1. Field of the Invention
The present invention relates to a method and an apparatus for detecting focusing errors in the recording or reproducing of information on recording media such as optical discs.
2. Related Background Art
Optical discs have recently been put into practice, and are being further developed, as high density, random access information memories with short access time, and in which recording or reproducing can be performed without contacting the recording medium.
Optical discs may be divided into three categories: (1) a read only type exclusively used for reproducing, such as a compact disc or a laser disc, for example, (2) a write once, read many times type in which information can only be recorded once but can be repeatedly reproduced, and (3) an erasable type in which recorded information can be erased and in which information can be repeatedly recorded or reproduced, such as optical discs of the phase change type, the magnetooptical type or the photochromic type.
In these optical discs, information is recorded by causing two minute regions having optical properties different from each other to correspond to information units "1" and "0". The minute regions are about 1 .mu.m in width and of various lengths. A shorter minute region is generally called a bit, a pit or a mark and is more prominent than a longer one. The longer region constitutes a ground or base of a recording track formed spirally or in coaxial circles. The bits, which are distributed along the tracks, can be constituted by projections, recesses, pits, magnetized regions having upward or downward magnetized directions, crystalline or noncrystalline regions, or regions having different colors.
When an optical disc is reproduced, the bits are irradiated by a light beam which is stopped down to a minimum and is focused to form a beam spot on the optical disc by an objective lens. The diameter of the smallest possible spot depends on the wavelength of the light beam. Since the wavelength of light obtained from a semiconductor laser is about 800 nm, the diameter of the smallest spot is, at the present time, about 1 .mu.m. The smallest usable bit size is determined by the diameter of the smallest spot, and the smaller the bit size, the higher the recording density that is possible.
For proper focusing of the beam spot, the objective lens must always be positioned at a constant distance from the optical disc. This distance is commonly equal to the focal distance of the objective lens. Thus, the operation for positioning the objective lens at a predetermined position is called focusing. If the distance between the objective lens and the optical disc is shorter or longer than the focal distance, the diameter of the spot increases, thereby decreasing the S/N ratio and increasing the rate of reading errors.
The optical disc may be displaced upwardly or downwardly, due, for example, to warping of the optical disc, or to vibration of the shaft that rotates the optical disc. It is very difficult to keep the distance between the objective lens and the optical disc constant.
As set forth hereinafter, the present invention solves this problem by causing the objective lens to follow the vertical displacement of the surface of the optical disc. (It is difficult to cause an entire optical head, also called an optical pickup, which has an optical system including the objective lens, a light source, a photodetector, etc., to follow the displacement of the optical disc, because the head is heavy.) In order to cause the objective lens to follow the displacement of the surface of the optical disc, it is first necessary to detect the deviation of the disc surface from the focal point. The deviation is called a focusing error.